Pretty embarrassing, right? Oh well, we all make stupid mistakes sometimes (or all the time) :withstupid:
If you don't see the issue with the above code, let me explain. The question was "Find the sum of all the multiples of 3 or 5 below 1000." The above solution does that, sort of - but it counts some numbers twice, for example 15. 15 is a multiple of both 3 and 5. Code revision time:

Perfect. It works! But... we did this in python for a reason. What's the point of using python if you don't even make the code pythonic? So, we can actually change this to be a pretty little one liner:

print(sum(i for i in range(1000) if i % 3 == 0 or i % 5 == 0))

Aaaaaand I'm sure there is something even cooler, but I'm satisfied with this solution.

Put that in your interpreter, (make sure you understand the solution), then head over to Project Euler and save your answer for problem 1!